The Offline tab lets you tell VPOP3 to move old archived messages to an alternative store, such as CD/DVD or external drive etc. VPOP3 will create a ZIP file containing the old messages. The archive index will still remember the messages, and the name of the ZIP file they are contained in, so you will still be able to search for messages on date, sender, subject, etc, but not on the message content.
Note that the ZIP file will contain messages starting from the oldest archived message until the Maximum Backup Size or the Backup Messages Older Than/Backup Messages before conditions are met. So, if you backup messages over 30 days old, and set a maximum backup size limit of 50MB, then VPOP3 will only create one ZIP file containing the oldest messages, which are at least 30 days old, until it reaches a size of 50MB. It won't create multiple ZIP files each of 50MB.
Once you press Start Backup VPOP3 will start creating the ZIP file, and you cannot create another ZIP file until the first one has been completed, or VPOP3 is stopped.
The archive creation can take a long time if the target file is large, because VPOP3 needs to create it in sections. The target size is after compression, so VPOP3 cannot tell how large a compressed message will be until it has actually been compressed. A 620MB ZIP may contain 100,000 message files, so this can take some time.
For possible error messages during offline archive creation see: Offline archive error messages